AC electro-hydro-dynamic (ACEHD) refers to the microfluidic flows induced in the vicinity of electrodes when an alternating current (AC) signal is applied. The term “AC signal” refers to a voltage that either alternates in polarity or varies periodically in amplitude. The AC signal induces periodically varying and non-uniform charges in the bulk fluid near the electrode (a.k.a. AC electrothermal effect) and/or in the electrochemical double layer (also known as double layer polarization) at the electrode surface (a.k.a. AC electroosmosis or ACEO). The varying and non-uniform charges produce migration of ions, and hence fluidic motion. ACEHD devices may be applied as particle traps, microfluidic pumps, mixers, and so forth.
ACEO devices, as a subset of ACEHD devices, are limited to fluids with conductivities lower than 80 mS/m. ACEHD devices can handle fluids with conductivities range from 1 μS/m to 2 S/m.
Many ACEO devices adopt planar interdigitated electrodes. Interdigitated electrodes are electrodes that are formed as two sets of opposing generally planar comb-like structures that have their “teeth” interlaced but not touching. When AC signals of opposite phase are applied across the two sets of such electrodes that are in contact with a fluid, electric fields are produced in the fluid in the region above and between adjoining teeth. These electric fields have components that are both normal and tangential to the electrodes. The tangential component of the field induces electro-osmotic fluid motion. Also, when an electric field is applied over a fluid body, energy is dissipated within by P=σErms2 (σ: the electrolyte conductivity), leading to temperature rise. Non-uniform temperature rise, i.e. temperature gradient ∇T, produces gradients in conductivity and/or permittivity as ∇ε=(∂ε/∂T)∇T, ∇σ=(∂σ/∂T)∇T, and further, ∇σ and ∇ε will generate mobile space charges, ρ, in the fluid bulk in AC fields. The space charges migrate under the influence of electric field and induce flow. Planar interdigitated electrodes have an effective range on the order of hundreds micrometers from the electrode surface.
AC electro-osmotic systems with interdigitated electrodes have generally short effective range of trapping bacteria and other particles. Also they are not effective for fluids with conductivity higher than 20 mS/m. So they have been used mainly to assist detection rather than collection, separation and remediation. Dielectrophoretic systems typically only collect contaminants having a minimum particle size greater than a few microns. What is needed therefore is a system for collecting large quantities of entrained particles with fewer limitations regarding minimum particle size, and fluid conductivity, thereby, for example, permitting effective collection of nanosize particles.